Which size of aggregate used in RCC structure?

The constraints for Nominal Maximum Size of Aggregate (NMSA) are follows

1. Cover to Concrete
2. Clear distance between reinforcements- 5 mm
3. One third of least lateral dimension

In general 20 mm is used as NMSA but use of 40 mm provides economy for substructures. For too congested structures 12.5 mm or 10 mm is also used. For PQC 31.5 mm and DLC 26.5 mm is considered. For mass concrete dams up to 150 mm is used. For shotcretes 8 mm is used.

It depends on the structure, whether there are steel reinforcement bars provided or it is plain concrete.

For concrete slabs with reinforcement as in bridge decks, roof slabs and columns, and prestressed concrete members, 16 – 20 mm is the MSA (maximum size of aggregate).

For roads, 38 – 40 mm MSA.

For retaining walls and abutments = 75 mm MSA.

For Concrete Dams – 75 mm near the face where there steel reinforcement provided and 150 – 200 mm MSA in the rest of the dam. Here concrete is poured by cranes from concrete buckets.

If the concrete is pumped by concrete pumps, then 20–25 mm MSA.

In ordinary structural concrete, the agg’s occupy 65 – 75% of the vol. of hardened mass.

• Fine agg’s: US std sieve (opening 4.75mm) passing & sieve (opening 0.075mm) retained.
• Coarse agg’s: US std sieve (opening 4.75mm) retained.

The coarse agg’s should be easily fit into the forms & between the r/f bars. For this purposes, it should not be longer than:

1. 1/5th of the narrowest side of the form.
2. 1/3rd of the depth of slab.
3. 3/4th of min dist between the r/f bars.

Usually 40mm, 20mm, 12.5mm, 10mm coarse agg’s are used in common practices.

1. 40mm may be used for massive foundation works.
2. For mass concreting purpose like dams up to 150mm agg’s may be used.